Selenium Never Forgets

Hugh G. White (G'Raffiti; reprinted with permission)

In the mid '30s, Chester Carlson invented a new method
of copying images onto paper. Prior to that time, only
a wet process similar to photography was used to make copies.

Carlson developed a dry process that transferred powdered
ink, called toner, from an optically induced image
on a negatively charged transfer device to a piece of
positively charged paper. The toner was then heated
which melted and fused it onto the paper.

In the process of perfecting his new copying technology,
Carlson experimented with belts and plates as the
transfer device. Neither of these worked very well.
Finally, Carlson set upon the idea of using a coated drum
as the transfer device. At first, in an attempt to imitate
the photographic process, he used silver compounds to
coat the drum. Eventually, Carlson chose selenium as the
coating. The process, now known as xerography (from the
Greek for "dry writing"), earned Carlson a patent -- and
now everyone knows about Xerox copiers.

Later, when Carlson was asked to identify the most
difficult part about inventing xerography, he stated that
the hardest problem was finding the proper coating. He
said, "The primary reason that we settled on selenium is
its unique crystal lattice and the way that it retains an
electrostatic charge indefinitely. We chose selenium
because it truly is the element that never forgets."

The preceding article was published in the April 1993
issue of Lithosphere, the official bulletin of the
Fallbrook [California] Gem and Mineral Society, Inc; Richard Busch
(Editor).

Permission to reproduce and distribute this material, in
whole or in part, for non-commercial purposes, is hereby granted
provided the sense or meaning of the material is not changed and
the author's notice of copyright is retained.

Last updated: 18 September 2002
http://geopress.rbnet.net/selenium.htm